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Politicians rapidly become accustomed to being pilloried as a matter of course by sizeable numbers of the population. Their customary ranking in the trustworthiness stakes alongside second-hand car salesmen and real estate agents is the penalty they must pay for putting themselves out there and having their thoughts, words and actions publicly judged on a daily basis.

My own experience, paradoxically, is that while politicians as a class are slated, individual politicians are often treated - on those many occasions when they meet the public - with exaggerated respect. But it remains the case that many politicians seem to try with some success to give their profession a bad name.

Politicians face two kinds of moral hazard; on the one hand, they face the same tests as the rest of us in meeting the standards that are normally required of everyone in their interaction with each other; but in addition they are required to make the correct responses to the demands that arise from the particular and public responsibilities they must discharge.

First, then, politicians cannot - or at least should not - seek any form of dispensation from the ordinary ethical requirements of honesty and straight dealing that are required of everyone in their normal dealings with others.

There is nevertheless a temptation in some quarters - and it is a temptation to which some politicians fall victim - to believe that the assertion that "all's fair in love and war" can be extended to politics.

I have never accepted that lying and cheating are acceptable because "it's just politics". If anything, the normal standards of behaviour should be higher for those who claim that they should be trusted with the power to make important decisions on behalf of all citizens.

Secondly, politicians also face the additional burden of ensuring that they use their special powers and responsibilities to the advantage of their constituents and not their own.

These matters are not always easy to judge or regulate, and can often depend on culturally different assessments of what is or is not acceptable.

In my years as a British MP, I lost count of the number of times on which, in return for help that I felt it might be my duty to give, I would be offered a gift (sometimes quite valuable) by a constituent - often from another culture - and have to explain politely that I could not accept it. But there are of course those politicians who are less concerned with such niceties - and that way lie "pork barrel" politics and corruption.

New Zealand is consistently rated as one of the countries where such behaviour is least prevalent. But we should be alert to instances where one or other or both of these moral traps seems to have ensnared one of our politicians, which is why the John Banks affair should ring alarm bells.

There are two aspects of the case that worry me but which have attracted little attention so far. First, we are told, in Kim Dotcom's sworn evidence, that he was asked by John Banks for anonymous donations because that would allow him to "help" Dotcom more effectively. This seems perilously close to an overt proposition that the mayoral candidate was willing to use his power, if elected, to offer differential "help" to a particular interest, and that this exercise would be aided if the financial help being solicited could be kept secret.

I am surprised that so few seem to have grasped the unacceptable nature of this proposed arrangement. The sale of favours by politicians should be anathema to any system of fair and open democracy.

The second issue is a mystery at the heart of the Prime Minister's continued defence of his ministerial colleague. Whatever one may think of John Banks, there will be surprise that the Prime Minister has maintained such indifference to the compelling evidence that his minister was less than truthful in his treatment of supposedly "anonymous" donations.

The explanation usually offered for the "see no evil" stance by the Prime Minister is that he needs John Banks' vote if he is to get his legislation through the House. But this doesn't wash. John Key could easily dismiss Banks from his government and avoid further damage to his own standing without jeopardising his wafer-thin parliamentary majority; Banks would still be an MP and able to vote for the government's legislation.

The fact that the Prime Minister has not taken this obvious course suggests strongly that there is an element in the situation of which we are not aware. That element can only be an implied threat from John Banks, along the lines of "as a minister, I'm obliged to vote for the government, but if I were reduced to the back benches, I could not guarantee to support any particular piece of legislation".

If this is the case, as I think it must be, John Banks has compounded his slipperiness with the truth by, in effect, making an implied threat to the Prime Minister.

We must hope that, in the interests of good government, the Prime Minister will have the courage to call his bluff.